The present invention relates to a method for opening a closed clutch arrangement of a drivetrain for a motor vehicle.
Methods of said type serve for carrying out a gearchange, in particular in a dual-clutch transmission, in as jerk-free a manner as possible.
Motor vehicles having an automatically actuated clutch and transmissions are generally known. During a gearchange, the automatic clutch interrupts the power flow between the engine and the drive wheels and conventionally operates with a high level of comfort, since said automatic clutch opens and closes in a comparatively smooth manner. In the case of automatic multi-step transmissions, use is also made of dual-clutch transmissions with two parallel separating clutches, with an interruption in tractive force being prevented in that, in the time overlap, a gear which is connected to a first separating clutch is disengaged and, at the same time, another gear is engaged by means of a second separating clutch.
A smooth opening and closing of the two separating clutches in order to prevent a shift jerk leads to long shift processes. A shortening of the shift duration by means of accelerated opening and closing of the separating clutches conventionally leads to a shift jerk and therefore to a less comfortable shift behaviour.
A precise gearchange by means of a time-overlapping opening and closing of two clutches requires a precise determination of an opening point of the clutch to be opened. Since the clutch is acted on with a permanently alternating engine torque, a precise determination of the opening point is difficult.
DE 101 50 597 A1 describes a method in which dead times during the actuation of the clutch arrangement are prevented, or the shift times are reduced, in that the clutch arrangement is held in such an actuating state that the torque which it can transmit corresponds to, or is slightly lower than, a torque output by the drive unit.
This means that the clutch is operated close to the opening point and is therefore permanently in a slip state, that is to say that the input rotational speed of the clutch is higher (in traction operation) or lower (in overrun operation) than the output rotational speed. Said state is referred to as micro-slip.
A disadvantage here is that the permanently slipping clutch leads to power losses in the drivetrain.
Furthermore, in the method from DE 10 150 597 A1, the correlation of the clutch pressure to the transmitted clutch torque is determined on the basis of characteristic maps. Said value pairs are determined on the basis of the slipping clutch. Said value pairs are however not necessarily current, such that the shift process can be initiated only with a small amount of precision.